A common characteristic of many stents in the market is longitudinal axis symmetry. Some of the stents, such as the Medtronic WIKTOR.TM. stents and the Medtronic InStent CARDIOCOIL.TM. stents, have a spiral structure. Such spiral structures have a cyclic helical coil element called "pitch." Other stents, such as the ACS MULTILINK.TM. stent, the Johnson & Johnson PALMAZ-SCHATZ.TM. stent, and the Medtronic Instent beSTENT.TM., are designed with a cyclic mesh element. Further stents such as the Johnson & Johnson Articulated Stent and the NIR stent have a double cyclic structure, that is, odd and even longitudinal elements where an odd element comes after an even element, and all the odd elements are identical and all the even elements are identical.
All the above stents have in common the characteristic of the uniformity of flexibility or durability for bending along the length of the stent.
Tests done on long stents loaded with bending stresses reveal a large bending moment in the central area of the stent. (See, for example, FIGS. 1a-1f.) Such stresses may cause fatigue-and even fracture at the center of the stent. On the one hand, significant stent flexibility is required during deployment while, on the other hand, significant rigidity is required in the expanded state of the stent. Since the bending moment increases from zero at the ends of the stent to a maximum value at the center, it has been determined that there is a need for a stent having a significant durability for bending at the exact place where the durability is required, that is, at the middle of the stent.